1. Field of the Invention
The present invention relates to improvements in a piezoelectric substrate of a structure having an ultra-thin resonating portion surrounded with a thick annular marginal portion formed integrally therewith, a piezoelectric resonating element having a conductive pattern including excitation electrodes and so on formed on the piezoelectric substrate, a piezoelectric resonator having the piezoelectric resonating element hermetically sealed in a package, a piezoelectric oscillator using the piezoelectric resonator, and a piezoelectric substrate wafer; more particularly, the invention concerns a technique which in the case of forming the resonating portion profiled with a concavity formed by etching in the surface of a piezoelectric substrate of an anisotropic crystal material, implements the optimum configuration of the resonating portion in conformity to ultraminiaturization of the piezoelectric substrate through utilization of an unetched portion (gentle slopes) on an inner wall of the annular marginal portion, and a technique which increases mass productivity by batch production while maintaining quality. Furthermore, the invention pertains to a method suitable for fine adjustments to the thicknesses of the resonating portions formed by the bottom portions of a plurality of concavities prepared by one operation in a piezoelectric substrate wafer, and a technique for providing the widest possible area for the resonating portion in a limited narrow piezoelectric substrate.
2. Background Art
[First Prior Art Example]
Surface-mount piezoelectric devices of the type having a crystal resonator or similar piezoelectric resonating element hermetically sealed in a package are used as a reference frequency source, a filter and so forth in communication equipment such as a portable telephone and a pager, and in electronic equipment such as a computer; and as these pieces of equipment become miniaturized, there is also growing a demand for miniaturization of the piezoelectric devices.
Furthermore, a piezoelectric oscillator for use as a surface-mount piezoelectric device has a structure in which a piezoelectric resonating element and parts forming an oscillation circuit are housed in a concavity formed in the top surface of the package body as of ceramics material and sealed therein by covering the open top of the concavity with a metal cover.
As the piezoelectric resonating element for use in such a piezoelectric device as mentioned above, there has been known a piezoelectric resonating element composed of: a piezoelectric substrate which has, for high-frequency operations, a thin resonating portion formed by the bottom of a concavity formed by removing part of the substrate surface and surrounded with a thick annular marginal portion integral therewith; input/output electrodes and a grounding electrode formed on top and bottom surfaces of the resonating portion (Pat. Laid-Open Gazette No. 9-055635).
FIGS. 15(a) and (b) are a perspective and a sectional view showing the configuration of an AT-cut crystal resonating element as an example of such a piezoelectric resonating element. The crystal resonating element, denoted generally by 100, is provided with: a crystal substrate 101 formed of an AT-cut crystal as an anisotropic piezoelectric crystal material; excitation electrodes 110 formed on both major surfaces of the crystal substrate; lead electrodes 111 extending from the excitation electrodes 110; and connecting pads 112 forming respective lead electrode terminating ends. The crystal substrate 101 has a construction in which an ultra-thin resonating portion 103 is formed by the bottom of a concavity 102 made by etching in one of two major surfaces of a rectangular, flat-shaped substrate body longer in the x-axis direction and the outer marginal edge of the resonating portion 103 is integrally held by a thick annular portion 104. That one side 104A of the annular marginal portion 104 lying in the x-axis direction is extended a predetermined length in the x-axis direction to form a jut-out portion 105. On one surface of the jut-out portion 105 the lead electrodes 111 are route thereto and the connecting pads 112 are disposed at the ends of the lead electrodes 111.
The reason for making the AT-cut crystal substrate 101 longer in the x-axis direction as mentioned above is that the propagation velocity of waves in the x-axis direction during excitation is approximately 1.2 times higher than the propagation velocity of waves in the x-axis direction; it is customary in the art to adopt such an x-axis long structure which is longer in the x-axis direction.
In the case of using chemical etching to form the concavity 102 in the crystal substrate 101, gentle slopes 104a and 104b of small inclination angles θ are left unetched on inner walls of the annular marginal portions 104 lying in the z-axis direction due to the property of the crystal as an anisotropic crystal material.
FIG. 15(c) is a sectional view showing the state in which the crystal resonating element 100 of the above-described structure is mounted in a surface-mount package 120, wherein the connecting pads 112 of the crystal resonating element 100 with the concavity 102 facing downward are electrically and mechanically fixed by conductive adhesive 122 to pads 121 disposed on the inner bottom of the package 120. The top opening of the package 120 is hermetically sealed with a metal cover 123.
Incidentally, in the case of mass-producing such crystal substrates 101 (or crystal resonating element 100) by batch production through use of a large-area piezoelectric substrate wafer, arrays of individual crystal oscillating elements 100 are laid out as shown in FIG. 16. That is, plural straight dicing grooves (dividing grooves) 131 are cut in a wafer 130 in a grid pattern so that they cross one another at right angles, and rectangular areas defined by the grooves ultimately become individual crystal substrates 101. Through etching of the wafer 130 by use of a predetermined etchant with a mask (a resist film) through which is exposed the crystal substrate surface where the concavity 102 will ultimately be formed, the gentle slopes 104a and 104b are left unetched on the inner walls lying in the z-axis direction corresponding to the crystal orientation in which the etching rate is low, as shown. Thereafter, the excitation electrodes 110, the lead electrodes 111 and the connecting pads 112 are formed in the individual substrate regions as by vapor deposition, after which the wafer is severed along the dicing grooves 131 into individual crystal resonating elements 100.
Incidentally, the crystal substrate to be housed in an ultraminiature package measuring 2.5×2.0 mm needs to be further shrunk to a size of less than 1.3×0.9 mm. On the other hand, in batch production using the wafer 130 it is necessary that the individual crystal substrates be closely spaced to enhance mass productivity by increasing the number of crystal substrates obtainable from each wafer, but in the fabrication of such ultraminiature crystal substrates as mentioned above the spacing w between the dicing groove 131 and each of three marginal edges of the concavity 102 is extremely narrow, making it difficult to provide a sufficiently broad and sufficiently strong annular marginal portion 104. Accordingly, in the case of cutting the wafer along the dicing grooves 131 by means of a dicing blade or similar cutting means, cracking readily occurs in the annular marginal portion 104 and the resonating portion 103, giving rise to a problem of sharp reduction in productivity.
Moreover, since the lead electrodes 111 extending from the excitation electrodes 110 formed on the top and bottom surfaces of the resonating portion 103, respectively, need to be routed along the inner wall of the steeply sloped one side 104A of the annular marginal portion 104 located in the x-axis direction as depicted in FIG. 15(a), the conductive traces are readily broken at sharp marginal edges.
Besides, as shown in FIG. 15(c), the connecting pads 112 are formed on the jut-out portion 105 contiguous to the side 104A with the steeply sloped inner wall and bonded to the pads 121 on the inner bottom of the package by use of the conductive adhesive 122, and consequently, the entire crystal resonating element structure is supported in a cantilever fashion; in this case, however, since the distance between the position where the connecting pads are bonded by the conductive adhesive 122 and the resonating portion 103 is short, stress due to the weight of the crystal resonating element is likely to be applied to the resonating portion 103 to distort it, causing resonance frequency variations.
[Second Prior Art Example]
FIGS. 17(a) and (b) are a sectional view of another conventional surface-mount crystal resonator and a sectional view taken on the line A—A, the resonator having a configuration in which the package 120 having housed therein the crystal resonating element 100 held in a cantilever fashion is hermetically sealed with the metal cover 123. On both sides of the jut-out portion 105 of the crystal substrate 101 there are formed two connecting pads 112a and 112b, respectively. In this instance, the connecting pad 112a facing toward the inner bottom of the package can easily be connected electrically and mechanically to the pad 121a opposite thereto by the conductive adhesive 122, but the connection of the other connecting pad 112b to the corresponding pad 121b in the package requires double coating of the adhesive since the former is formed on the flat surface of the crystal substrate. The double coating of the adhesive involves first coating of the adhesive between the pad 121b and the underside of the crystal substrate and second coating of the adhesive to interconnect the upper connecting pad 112b and the adhesive coated first.
When coated twice, however, the adhesive 122 partly protrudes upward of the connecting pad 112b, and to prevent it from contacting the underside of the metal cover 123, it is necessary to increase the height of the outer peripheral wall of the package 120. This constitutes an obstacle to a reduction in profile of the package and ignores the demand for miniaturization.
As a solution to this problem, it is conventional to employ such a structure as shown in FIG. 17(c), in which a concave notch 140 (140a, 140b) deposited over the entire area of its inner wall with a conductive film is formed in the edge face of the substrate adjacent the marginal edge of the upper connecting pad 112b to establish electrical connections between the conductive film on the inner wall of the concave notch 140b and the connecting pad 112b on the top of the substrate, whereas on the underside of the substrate there is formed a connecting pad 112b′ for electrical connection with the conductive film on the inner wall of the concave notch 140b. With this structure, the lower connecting pad 112b′ and the pad 121b on the inner bottom of the package are connected via the conductive adhesive 122, establishing electrical connections between the upper connecting pad 112b and the pad 121b by single coating of the adhesive.
Such a concave notch as mentioned above is formed using the procedure as shown in FIG. 17(d): making small rectangular holes in each crystal substrate 101 from both of its top and underside surfaces by chemical etching using a mask (resist film) for the large-area piezoelectric substrate wafer 130; interconnecting the both small rectangular holes to form a through hole 140H; depositing the conductive film all over the inner wall of the through hole; and severing the wafer along the dicing grooves 131 into individual crystal substrates. However, the diameter (width) of each through hole 140H to be formed within the width of the connecting pad 112b on the ultraminiature crystal resonating element measuring, for instance, less than 1.3×0.9 mm inevitably becomes as small as on the order of μ—this causes frequent occurrence of insufficient etching that does not completely interconnect the small rectangular holes made in each crystal substrate from its top and underside surfaces. On the other hand, since the through hole 140H forming the concave notch 140 is formed in a narrow area of the connecting pad 112b of a limited area, the diameter of the hole is also limited accordingly. It is particularly difficult to form two through holes in one marginal edge face of each piezoelectric substrate of an extremely small area.
Accordingly, there has been a strong demand for solving the problem of low yields of ultraminiature crystal resonating elements caused by insufficient chemical etching of the large-area piezoelectric substrate wafer 130 to form therein the through holes 140H which are ultimately used as the concave notches 140.
Incidentally, the reason for providing the pair of concave notches 140 in the edge face of each crystal substrate 101 is that the one concave notch 140b is to establish electrical connections between the upper connecting pad 112b and the pad 121b on the package as referred o above, whereas the other concave notch 140a is to provide on the top surface of the crystal substrate the upper connecting pad 112a′ electrically connected to the lower connecting pad 112a. With such an arrangement, measurement of characteristics of each individual crystal resonating element formed on the wafer 130 can be done with probe pins of a measuring instrument held from the same direction against the two connecting pads 112b and 112a′ on the top of the crystal substrate or the two connecting pads 112a and 112b′ on the underside of the substrate. The reason for this is that it is most efficient to conduct the measurement with the probe pins held against the two connecting pads on the same surface of the substrate.
Besides, the crystal resonating element 100 is not always mounted in the package with the concavity oriented downward as shown in FIG. 17(a), but it may also be held upward. Hence, the provision of the two connecting pads on either side of the substrate enables one crystal resonating element 100 to be mounted in the package in an arbitrary orientation.
[Third Prior Art Example]
In the formation of concavities by chemical etching in individual piezoelectric substrate regions on a sheet-like piezoelectric substrate wafer with a plurality of piezoelectric substrates arranged in a matrix form, it is difficult to make uniform the thicknesses of all ultrathin resonating portions formed by concavity bottom portions. To obviate this problem, it is customary in the prior art to premeasure the depth of each concavity, that is, variations in the thicknesses of the resonating portions in the respective concavities, and to conduct an adjustment operation using an etchant for each concavity to make fine adjustment to the thickness of the resonating portion having not reached a predetermined value.
FIGS. 18(a) and 18(b) are diagrams for explaining a conventional fine adjustment method for each concavity, according to which the concavities 102 are formed by simultaneously etching only those wafer surface areas exposed through apertures of a mask (resist film) covering the one major surface of the piezoelectric substrate wafer 130, though not shown. Since such one operation by etching does not make uniform the thicknesses of the resonating portions 103 formed by the bottom portions of the concavities 102, the thicknesses of the resonating portions 103 of the concavities 102 are premeasured, and then etching is carried out for each concavity after a guide mask with apertures arranged in a grid pattern, such as denoted by reference numeral 150, is mounted on the wafer 130 and held in close contact with the wafer surface areas between adjacent concavities. That is, the guide mask 150 has a plurality of apertures 152 of a rectangular or some other shape formed through a sheet of resin, for instance, with a predetermined pitch; the apertures 152 of the shape matching the plan configuration of the concavities 150 are defined by adjacent partitioning parts 151 intersecting in a grid pattern. The guide mask 150 is fixed to the wafer 130 all over it with the partitioning parts 151 held in close contact with the wafer surfaces around the concavities 102 as shown in FIG. 18(b). Then, the concavities are sequentially filled with proper amounts of etchant 155 for different periods of time precalculated therefor in decreasing order of thickness of the resonating portion. At a point in time all the resonating portions have been etched to a predetermined thickness, the entire wafer assembly is cleaned up to remove therefrom the etchant.
Incidentally, miniaturization of the apertures 152 of the guide mask 150 is limited due to limitations imposed on machining techniques; and an achievable minimum size is such as depicted in FIG. 18(b). Accordingly, in order to make fine adjustments to the thicknesses of the resonating portions of miniature concavities 102 in the wafer 130 having more miniature piezoelectric substrates by individual etching as shown in FIG. 18(c), there is no choice but to use the guide mask 150 prepared for large concavities. Alternatively, to permit accurate dropwise filling of the concavities with the etchant, the apertures 152 need to be of such a size as shown in FIG. 18(b) at minimum. When such a guide mask is used, the concavities 102 and the dicing grooves 131 are exposed through the apertures 152 defined by adjacent partitioning parts 151, as depicted in FIG. 18(b). In this instance, when each aperture 152 is filled with the etchant, excess etchant overflowing the concavity penetrates into the dicing grooves 131 and unnecessarily etches away those regions undesired to etch, incurring a decrease in the mechanical strength of the regions concerned. Moreover, there is a fear that the surface tension of the etchant 155 filling the concavity 102 prevents it from making full contact with the entire area of the bottom of the concavity, leaving therein unetched portions 156 as shown in FIG. 18(d), and hence resulting in the individual etching becoming unsuccessful.
As described above, in the case of individual etching of the concavities to remove variations in the depths of the concavities formed by batch operation in the wafer, the limitations on the size of the apertures of the guide mask 150 incur the possibility of unnecessary etching or poor etching of the resonating portion.
[Fourth Prior Art Example]
FIG. 19 is a sectional view showing the configuration of an AT-cut crystal substrate as an example of the piezoelectric substrate. The crystal substrate 101 is made of an AT-cut crystal as an anisotropic piezoelectric crystal material, and the crystal substrate 101 has formed in both major surfaces thereof concavities 102a and 102b that are symmetrical about a point to each other. That is, the crystal substrate 101 has the concavities 102a and 102b formed therein by etching a rectangular flat-shaped substrate body through masks (resist) 160 covering its both major surfaces in such a manner that the bottom panel common to the concavities 102a and 102b form an ultrathin resonating portion 103 integrally with the thick marginal portion 104. Due to a difference in etching rate between the z- and x-axis directions, the inner walls 104a and 104b of two sides of the annular marginal portion 104 lying in the z-axis direction slope more gently than the other inner walls lying in the x-axis direction. In addition, the both inner walls 104a and 104b differ in inclination angle.
In etching, however, when the masks 160 having the apertures of the same shape are mounted on both sides of the crystal substrate 101 in alignment with each other, the z-axis side inner walls 104a and 104b of the concavities 102a and 102b bear such symmetric positional relationship as shown, in consequent of which edges 102′ and 102b′ of the bottom surfaces of the respective concavities 102a and 102b are not in aligned relation. Since the edges 102a′ and 102b′ of the bottom surfaces of the concavities 102a and 102b are thus displaced in the z-axis direction relative to each other, the bottom surfaces of the two concavities are not directly opposite, decreasing the area of each resonating portion 103 and consequently the effective thin region (the effective vibrating region). This raises a problem of deteriorated characteristics of the crystal resonating elements with electrodes and so forth formed on such crystal substrates. In particular, further miniaturization of the piezoelectric substrate will increase the severity of such a glitch.
[Fifth Prior Art Example]
A description will be given of, with reference to FIGS. 20 and 21, of a conventional method for manufacturing the crystal resonating element with an ultrathin resonating portion. This is the manufacturing method that the inventor of present invention disclosed in Technical Report of the Institute of Electronics, Information and Communication Engineers of Japan, “UHF-Band Crystal Resonator Using Fundamental Wave,” (Technical Report of IEICE US98-27, EMD98-19, CPM98-51, OME98-49 (1998-07), Corporation-Institute of Electronics, Information and Communication Engineers of Japan).
FIG. 20 is a flowchart of a crystal resonator manufacturing process, and FIGS. 21(a) to (d) are longitudinal-sectional views showing the crystal resonator in an etching process, the chain double-dashed lines X in FIGS. 21(a) to (d) being imaginary lines indicating the thickness of the resonating portion at the end of four stages of the chemical etching process.
In a crystal resonator for fundamental wave vibration in the UHF or higher band, for instance, since the amount of change in frequency with respect to the amount of change in wafer thickness is large, the thickness of the crystal wafer is adjusted by four-stage chemical etching steps 203 through 206 to obtain the resonating portion 103 that is excitable at the desired fundamental-wave resonance frequency.
The manufacturing process begins with polishing the major surface of the crystal wafer (step 200), followed by vacuum depositing a gold/chromium film on the polished major surface (step 201). In view of a tradeoff between the mechanical strength of the wafer and the amount of etching, let it be assumed that the crystal wafer is 80 micrometers (m) thick. The gold/chromium film is selectively removed by photolithography to form a mask pattern for etching (step 202).
Thereafter, first main etching (step 203) through second fine-adjustment etching (step 206) processes are performed as described below. In the first main etching process (step 203), as depicted in FIG. 21(a), a crystal wafer 221 with a mask pattern for etching 224 formed thereon is subjected to wet etching to etch away the regions of the wafer underlying apertures of the mask pattern to form concavities as resonating portions 222a and 223a that resonate in the VHF band, for example, at 155 MHz. In practice, however, since the thicknesses of the resonating portions 222a and 223a differ due to wafer etching errors or the like, the resonance frequencies of the resonating portions 222a and 223a are measured.
And, in the first fine-adjustment etching process (step 204) shown in FIG. 21(b), an etchant is added dropwise to the respective concavities for different periods of time based on the measured resonance frequencies, by the technique disclosed, for example, in Pat. Laid-Open Gazette No. 6-021740, by which the thicknesses of the resonating portions 222a and 223a are individually adjusted so that their resonance frequencies be come as desired. Moreover, as depicted in FIG. 21(c), in second main etching process (step 205) the wafer is further subjected to wet etching to form resonating portions 222c and 223c each having a thickness of about 2.2 μm that corresponds to a resonance frequency in the desired UHF band, for instance, at 760.9 MHz. Then the resonance frequencies of the resonating portions are measured again, and in second fine-adjustment etching shown in FIG. 21(d) dry etching is carried out for each of the resonating portions 222c and 223c based on their measured frequencies so that they resonate at desired frequencies. After this, gold/chromium is vacuum deposited all over both major surfaces of the wafer (step 207), then electrode patterns are formed thereon (step 208), and the wafer is severed into the crystal resonating elements 100 (step 209). The crystal resonating elements 100 are each mounted in a package, then connected thereto by bonding or bumps (step 210), and sealed therein after being subjected to final frequency adjustment (step 211, 212).
The second fine-adjustment etching process (step 206) 114 is performed by dry etching of low etching rate for high-precision individual adjustment to obtain the thickness of approximately 2.2 μm which corresponds to the desired resonance frequency of 760.9 MHz.
FIG. 22 is a longitudinal section view showing the working of the resonating portion; when the afore-mentioned four-stage (step 203 through step 206) chemical etching for forming a resonating portion 232d from only one direction (indicated by the arrow), that is, from the direction of the opening of the concavity, the area of the resonating portion 232a on the side of the opening of the concavity gradually decreases and the area of the resonating portion 232d becomes extremely small due to the dependence of the etching rate on the crystal orientation, and consequently, a vibrating region 232h of the resonating portion 232d becomes extremely narrow than a desired value. For example, in the case of obtaining a resonator whose fundamental frequency is 760.9 MHz, the thickness of the resonating portion is about 2.2 μm, and if the thickness of the crystal wafer is set at 80 μm from the viewpoint of its mechanical strength, then it is etched to a depth of around 77.8 μm to form a concavity. Then, even if the opening of the concavity is 0.7×0.55 millimeters (mm), the area of the vibrating region 222h is approximately 0.25×0.15 mm that is smaller than the desired area. For example, when the oscillation frequency is 622.3 MHz, the desired size of the vibrating region 222h is required to be in the range of 0.5 to 0.75×0.3 to 0.45 mm that is twice to three times (taking into account variations caused during manufacturing process) larger than the size of the electrode to be formed in the vibrating region 232h (an ellipse of a size measuring a longer diameter 0.25×a shorter diameter 0.15 mm).
Furthermore, the slope between the top surface of the annular marginal portion 232b and the vibrating region 232h becomes so wide that a lead (not shown) formed on the slope becomes long, giving rise to a problem that the resistance or parasitic impedance of the lead increases.
In the second main etching process (step 205) the etchant used is a low-temperature ammonium hydrogen fluoride saturated solution, which prevents overetching but is low in working efficiency because of low etching rate; since the second fine-adjustment etching (step 206) is performed by dry etching of low etching rate for implementing high-precision adjustment, there is a problem of etching damage by crystal defects, contamination with an impurity, or the like.
Furthermore, supply control (flow rate and pressure) of an etching gas for dry etching has so high a correlation with uniformity of etching that the number of etching gas supply holes and their size must be changed for each operation; hence, it is difficult to obtain the optimum conditions for etching.
Besides, since the etching process shown in FIG. 20 is followed by the gold/chromium vapor deposition step 207 for vapor depositing the conductive film that will ultimately form a main electrode film 110 and then by the final frequency adjustment process (step 211) for making high-precision frequency adjustments by vapor deposition or sputtering, the thickness adjustment by the etching process needs only to make adjustments to such an extent as to allow compensation in the final frequency adjustment process (step 211), and hence the etching scheme in this process is more than required.
Furthermore, the combined use of wet etching and dry etching inevitably leads to complication of the manufacturing process and an increase in capital investment, constituting an obstacle to bringing down costs of UHF-band crystal resonators.